int maxProfit(vector<int> &prices) {
	int buy1 = -prices[0];
	int buy2 = -prices[0];
	int sell1 = 0, sell2 = 0;
	for (int i = 1; i < prices.size(); i++) {
		buy1 = max(buy1, -prices[i]);
		sell1 = max(sell1, buy1 + prices[i]);
		buy2 = max(buy2, sell1 - prices[i]);
		sell2 = max(sell2, buy2 + prices[i]);
	}
	return sell2;
}
